To extend the life of a tool, conventionally cutting oil (a coolant) is typically supplied to the tool during machining by a machine tool.
An external oil feeding method, in which the cutting oil is ejected toward the tool from an externally provided nozzle, is employed conventionally as a cutting oil supply method. With the external oil feeding method, however, the cutting oil ejected from the nozzle may be blocked by a work piece so as to be unable to reach a work part, and it is therefore difficult to supply the cutting oil effectively. Further, the work piece and the nozzle may interfere with each other, and therefore the nozzle must be provided in a position far removed from the work piece, making it difficult to supply the cutting oil to the work part precisely. Moreover, with the popularization of machine tools such as machining centers, in which tools are exchanged by an ATC (Automatic Tool Changer) during machining, it has become necessary to modify a cutting oil supply position for each tool, but the external oil feeding method, in which the position and orientation of the nozzle are fixed, is unable to respond to this requirement. Hence, the external oil feeding method has been replaced by a cutter through method or a gap through method, to be described below, and it is now rare to use the external oil feeding method in a machine tool.
In the cutter through method, an oil feeding hole that opens onto a tool tip end is provided in a tool interior, and the cutting oil is supplied to the work part through the oil feeding hole (see Patent Documents 1 and 2).
In the cutter through method, however, the oil feeding hole opens onto the tip end of the tool, and therefore, in a tool that cuts a work piece using a blade portion on a tool outer peripheral surface, such as a face mill or an end mill, the blade portion cannot be lubricated and cooled efficiently. Further, since the oil feeding hole must be provided in the tool interior, the tool becomes expensive. Moreover, when the tool has a small diameter, it is extremely difficult to form the oil feeding hole in the tool interior.
In the gap through method, meanwhile, a gap nut is attached to a tip end of the tool holder, and the cutting oil is ejected through a gap between the gap nut and a tool outer periphery. Therefore, the gap through method can be used with tools not having an internal oil feeding hole.
Patent Document 3, for example, describes a tool holder that is compatible with the gap through method. In this tool holder, a gap nut having a spiral groove on an inner peripheral side thereof is attached to a tip end, and cutting oil is ejected through a gap between the gap nut and the tool outer periphery. As a result, the cutting oil ejected so as to travel around the spiral groove of the gap nut is supplied to the work part efficiently along a flank of the tool.
Further, Patent Document 4, although not related to a method of feeding oil to a tool holder, describes a drive spindle employing a traction drive method in which rotation input from a transmission shaft coupled to a spindle of a machine tool is increased in speed by a traction transmission mechanism and then transmitted to a main shaft. The traction transmission mechanism is constituted by a combination of a planetary roller and a sun roller such that rotation is transmitted from the planetary roller, which revolves together with the spindle, to the sun roller, which is coupled to the main shaft. A working tool attachment portion is provided on a tip end of the main shaft into which rotation is input from the transmission shaft via the traction transmission mechanism, and a tool (a grinding wheel, for example) is attached to the working tool attachment portion.
Further, the drive spindle described in Patent Document 4 is provided with a cooling device for cooling the traction transmission mechanism and a main shaft bearing. The cooling device includes cooling jackets provided respectively on an outer periphery of the traction transmission mechanism and an outer periphery of the main shaft bearing, and a cooling medium passage extending from a location on an outer periphery of the transmission shaft to the cooling jacket provided on the main shaft bearing via the cooling jacket provided on the traction transmission mechanism. A cooling medium flowing into the cooling medium passage passes through the cooling jacket of the traction transmission mechanism and the cooling jacket of the main shaft bearing, and is then ejected through an ejection port formed in a bearing retainer plate that presses an outer race of the main shaft bearing.    Patent Document 1: Japanese Patent Application Publication No. 2009-6435    Patent Document 2: Japanese Patent Application Publication No. H4-176538    Patent Document 3: Japanese Patent Application Publication No. 2003-1545    Patent Document 4: Japanese Utility Model Application Publication No. H3-123657
In the gap through method, however, the ejected cutting oil spreads due to centrifugal force, and therefore the cutting oil cannot be supplied efficiently to the work part. Particularly when the tool is rotated at high speed, a large amount of heat is generated by the work part, and therefore a large amount of cutting oil is required to cool the tool. However, it is difficult to supply a sufficient amount of cutting oil to the work part due to the effect of the centrifugal force.
In the drive spindle employing the traction drive method, described in Patent Document 4, the cooling medium is ejected through the ejection port positioned on the outer periphery of the main shaft, but the ejection port is provided in a position removed from the tool attached to the working tool attachment portion, which is positioned on the tip end side of the main shaft. The reason for this is that in order to realize the function of the traction transmission mechanism for increasing the speed of the rotation input from the spindle side of the machine tool and then transmitting the rotation to the main shaft side, the traction transmission mechanism must be provided between the spindle of the machine tool and the main shaft having the tool attached to the tip end thereof. It is therefore difficult to supply the cooling medium to the work part precisely through the ejection port located far from the tool.
Further, in the drive spindle described in Cited Document 4, to ensure that rotation can be transmitted from the planetary roller to the sun roller in the traction transmission mechanism, a housing provided on an outer periphery of the planetary roller must be prevented from co-rotating therewith. If co-rotation of the housing is not prevented, the housing rotates as the planetary roller revolves such that torque is not transmitted to the sun roller when the planetary roller revolves around the sun roller, and as a result, the sun roller does not rotate. Hence, in the drive spindle described in Patent Document 4, a whirl-stop pin is inserted into an upper surface of the housing to prevent the housing from co-rotating. In other words, in Patent Document 4, the housing is prevented from co-rotating to ensure that rotation can be transmitted from the planetary roller to the sun roller in the traction transmission mechanism, but not to reduce the effect of the centrifugal force exerted on the cooling medium ejected through the ejection port formed in the bearing retainer plate.
Hence, Patent Document 4 teaches that co-rotation of the housing is to be prevented in order to realize the original functions of the traction transmission mechanism only when the traction transmission mechanism is provided between the spindle of the machine tool and the main shaft having the tool attached to the tip end thereof. Accordingly, Patent Document 4 provides no description of a solution with which the cutting oil can be ejected from a position close to the work part and supplied to a desired position without being affected by centrifugal force.